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International audience; The nitrogen-vacancy (NV) center in 3C-SiC, the analog of the NV center in diamond, has recently emerged as a solid-state qubit with competitive properties and significant technological advantages. Combining first-principles calculations and magnetic resonance spectroscopy we provide thorough insight in its magneto-optical properties. By applying resonantly excited electron paramagnetic resonance spectroscopy, we identified the zero-phonon absorption line of the 3 A 2 → 3 E transition at 1289 nm (within the telecom Oband) and measured its phonon sideband, the analysis of which reveals a Huang-Rhys factor of S = 2.85 and a Debye-Waller factor of 5.8 %. The low temperature spin-lattice relaxation time was found to be exceptionally long (T 1 = 17 s at 4 K). All these properties make NV in 3C-SiC a strong competitor for qubit application. In addition, the strong variation of the zero-field splitting in the range of 4K to 380K allows its application for nanoscale thermal sensing.

Silicon carbide (SiC) has become a key player in realization of scalable quantum technologies due to its ability to host optically addressable spin qubits and wafer-size samples. Here, we have demonstrated optically detected magnetic resonance (ODMR) with resonant excitation, and clearly identified the ground state energy levels of the NV centers in 4H-SiC. Coherent manipulation of NV centers in SiC has been achieved with Rabi and Ramsey oscillations. Finally, we show the successful generation and characterization of single nitrogen vacancy (NV) center in SiC employing ion implantation. Our results are highlighting the key role of NV centers in SiC as a potential candidate for quantum information processing.

We report the enhancement of the optical emission between 850 and 1400 nm of an ensemble of silicon mono-vacancies (VSi), silicon and carbon divacancies (VCVSi), and nitrogen vacancies (NCVSi) in an n-type 4H-SiC array of micropillars. The micropillars have a length of ca. 4.5 μm and a diameter of ca. 740 nm, and were implanted with H+ ions to produce an ensemble of color centers at a depth of approximately 2 μm. The samples were in part annealed at different temperatures (750 and 900 °C) to selectively produce distinct color centers. For all these color centers we saw an enhancement of the photostable fluorescence emission of at least a factor of 6 using micro-photoluminescence systems. Using custom confocal microscopy setups, we characterized the emission of VSi measuring an enhancement by up to a factor of 20, and of NCVSi with an enhancement up to a factor of 7. The experimental results are supported by finite element method simulations. Our study provides the pathway for device design and fabrication with an integrated ultra-bright ensemble of VSi and NCVSi for in vivo imaging and sensing in the infrared.

Deep and shallow electronic states in undoped and Si-doped e-Ga2O3 epilayers grown by MOVPE on c-oriented Al2O3 were investigated by cathodoluminescence, optical absorption, photocurrent spectroscopy, transport measurements, and electron-paramagnetic-resonance. Nominally undoped films were highly resistive, with a room temperature resistivity varying in the range 107- 1013 Ωcm depending on the carrier gas used during growth. Films grown with He carrier were generally more resistive than those grown with H2 carrier and exhibited a Fermi level located at about 0.8 eV below the conduction band edge, which tends to shift deeper with temperature. This can tentatively be attributed to the combined action of deep donors (probably carbon impurities and oxygen vacancies) and deep acceptors (Ga vacancies and related complexes), which compensate residual shallow donors. There are strong experimental hints that nitrogen also behaves as deep acceptor. Room temperature resistivity as low as 0.42 Ωcm and electron concentrations around 1018 cm−3 were obtained by silicon doping. Si was confirmed to act as shallow donor with sufficiently high solubility. A variable range hopping conduction was observed in a wide temperature interval in the n-type layers, and compensation by native acceptors also plays a major role on conduction mechanisms. Previous evaluations of curvature and anisotropy of the conduction band are confirmed, which allows for the estimation of the electron effective mass. The present experimental data are discussed considering the theoretical predictions for point defect formation in the e-polymorph as well as literature data on extrinsic and intrinsic defects in β-Ga2O3.

Octahedral molecular sieves (OMS) are built of transition metal-oxygen octahedra that delimit sub-nanoscale cavities. Compared to other microporous solids, OMS exhibit larger versatility in properties, provided by various redox states and magnetic behaviors of transition metals. Hence, OMS offer opportunities in electrochemical energy harnessing devices, including batteries, electrochemical capacitors and electrochromic systems, provided two conditions are met: fast exchange of ions in the micropores and stability upon exchange. Here we unveil a novel OMS hexagonal polymorph of tungsten oxide called h’-WO3, built of (WO6)6 tunnel cavities. h’-WO3 is prepared by a one-step soft chemistry aqueous route leading to the hydrogen bronze h’-H0.07WO3. Gentle heating results in h’-WO3 with framework retention. The material exhibits an unusual combination of 1-dimensional crystal structure and 2-dimensional nanostructure that enhances and fastens proton (de)insertion for stable electrochromic devices. This discovery paves the way to a new family of mixed valence functional materials with tunable behaviors.

Octahedral molecular sieves (OMS) are built of transition metal-oxygen octahedra that delimit sub-nanoscale cavities. Compared to other microporous solids, OMS exhibit larger versatility in properties, provided by various redox states and magnetic behaviors of transition metals. Hence, OMS offer opportunities in electrochemical energy harnessing devices, including batteries, electrochemical capacitors and electrochromic systems, provided two conditions are met: fast exchange of ions in the micropores and stability upon exchange. Here we unveil a novel OMS hexagonal polymorph of tungsten oxide called h’-WO3, built of (WO6)6 tunnel cavities. h’-WO3 is prepared by a one-step soft chemistry aqueous route leading to the hydrogen bronze h’-H0.07WO3. Gentle heating results in h’-WO3 with framework retention. The material exhibits an unusual combination of 1-dimensional crystal structure and 2-dimensional nanostructure that enhances and fastens proton (de)insertion for stable electrochromic devices. This discovery paves the way to a new family of mixed valence functional materials with tunable behaviors., Thanks to their versatile redox behaviors, octahedral molecular sieves show promise in a range of electrochemical applications. Here the authors report a hexagonal polymorph of tungsten trioxide, an octahedral molecular sieve that exhibits fast proton (de)insertion for electrochromic devices.

𝛽-Ga2O3 has recently received attention as an ultra-wide-band-gap material, promising as transparent conductor and for high-power device applications. In this talk, we review some recent computational work on the electronic band structure and it defects. The electronic band structure was calculated using the quasiparticle self-consistent GW method including a correction of the screened Coulomb interaction W for electron-hole interaction and lattice-polarization effect. The selection rules of the optical transition help understand the anisotropy of the optical absorption.1 Among the point defects, which have already been discussed in various papers,2 we here focus on the two types of Ga–vacancies, which can occur on both the tetrahedral and octahedral Ga site. Electron paramagnetic resonance (EPR) related to the Ga-vacancy has been reported.3,4 Here we present first-principles calculations of the g-factor and hyperfine structure, which allow us to identify the EPR spectrum observed after high energy particle irradiation, with the tetrahedral Ga vacancy in the 2- charge state. Modification of the EPR spectrum under low temperature photoexcitation are explained by the following model. The EPR spectrum of the tetrahedral Ga vacancy shows superhyperfine interaction with two Ga atoms which are nearest neighbors to the oxygen on which the defect localizes in the q=2- charge state. The g-tensor has its largest deviation from the free-electron value for the crystallographic b-direction in which the defect Ga-O-Ga complex is oriented. According to Ref. 2, the tetrahedral Ga vacancy has its 2-/3- transition level closer to the conduction band minimum than the octahedral one. Upon illumination, this defect may become inactivated while the octahedral one is activated. The latter has a different orientation of its main largest g-tensor component but similar superhyperfine splitting with two Ga atoms. These computational findings explain the corresponding experimental observations. 1 Amol Ratnaparkhe and Walter R. L. Lambrecht, Appl. Phys. Lett. 110, 132103 (2017) 2 Peter Deak, Quoc Duy Ho, Florian Seemann, Balint Aradi, Michael Lorke, and Thomas Frauenheim, Phys. Rev. B 95, 075208 (2017) and references therein. 3 B. E. Kananen , L. E. Halliburton , K. T. Stevens , G. K. Foundos , and N. C. Giles, Appl. Phys. Lett. 110, 202104 (2017) 4 H.Jurgen von Bardeleben, unpublished

A combined experimental/theoretical study of the EPR in irradiated $\beta$-Ga$_2$O$_3$ is presented. Four EPR spectra, two $S=1/2$ and two $S=1$, are observed after high-energy proton or electron irradiation. One of the S=1/2 spectra (EPR1) can be observed at room temperature and below and is characterized by the spin Hamiltonian parameters $g_b=2.0313$, $g_c=2.0079$, $g_{a*}=2.0025$ and a quasi isotropic hyperfine interaction with two equivalent Ga neighbors of $~\sim$14 G on $^{69}$Ga. The second (EPR2) is observed after photoexcitation (with threshold 2.8 eV) at low temperature and is characterized by $g_b=2.0064$, $g_c=2.0464$, $g_{a*}=2.0024$ and a quasi isotropic hyperfine interaction with two equivalent Ga neighbors of 10 G. A spin $S=1$ spectrum with a similar g-tensor and a 50\% reduced hyperfine splitting accompanies each of these, which is indicative of a defect of two weakly coupled $S=1/2$ centers. DFT calculations of the magnetic resonance fingerprint of a wide variety of native defect models are carried out to identify these EPR centers in terms of specific defect configurations. The EPR1 center is proposed to correspond to a complex of two tetrahedral $V_\mathrm{Ga1}$ with an interstitial Ga in between them. This model was previously shown to have lower energy than the simple tetrahedral Ga vacancy and has a $2-/3-$ transition level higher than other $V_\mathrm{Ga}$ related models, which would explain why the other ones are already in their diamagnetic $3-$ state and are thus not observed if the Fermi level is pinned approximately at this level. The EPR2 spectra are proposed to correspond to the octahedral $V_\mathrm{Ga2}$. Models based on self-trapped holes and oxygen interstitials are ruled out because they would have hyperfine interaction with more than two Ga nuclei and because they can not support a corresponding $S=1$ center.

Proton irradiation of both n-type and semi-insulating bulk samples of β-Ga2O3 leads to the formation of two paramagnetic defects with spin S = 1/2 and monoclinic point symmetry. Their high introduction rates indicate them to be primary irradiation induced defects. The first electron spin resonance (EPR1) has a g-tensor with principal values of gb = 2.0313, gc = 2.0079, and ga* = 2.0025 and quasi-isotropic superhyperfine interaction of 13G with two equivalent Ga neighbors. Under low temperature photoexcitation, this defect is quenched and replaced by a different metastable spin S = 1/2 center of comparable intensity. This second defect (EPR2) has similar principal g-values of gb = 2.0064, gc = 2.0464, and ga* = 2.0024 and shows equally superhyperfine interaction with two equivalent Ga atoms. This EPR2 defect is stable up to T = 100 K, whereas for T > 100 K the initial defect is recovered. Density functional theory calculations of the spin Hamiltonian parameters of various intrinsic defects are carried out using the gauge including projector augmented wave method in order to determine the microscopic structure of these defects. The intuitive models of undistorted gallium monovacancies or self-trapped hole centers are not compatible with neither of these two defects.

Previous Electron Paramagnetic Resonance (EPR) studies identified the carbon dangling bond center as the main paramagnetic interface defect in 3C, 4H, 6H-SiC/SiO2, We demonstrate that this defect, called P-bC center, can be passivated by forming gas annealing at 400 degrees C. We have measured the PbC density at annealed 4H- and 3C-SiC/SiO2 interfaces and attributed its reduction to the transformation of the dangling bonds into EPR inactive C-H bonds. We have also studied the reverse phenomenon occurring during vacuum annealing at temperatures ranging from 600 degrees C up to 1000 degrees C and have determined a dissociation energy of approximate to 4.3 eV for the 3C and 4H polytypes.

6H-SiC single crystalline substrates were implanted at room temperature with 2 MeV Al2+ ions to fluences from 2x10(14) Al-2, cm(-2) to 7x10(14) Al2+ cm(-2) and with different current densities (from 6.6 to 33x10(10) Al2+ cm(-2) s(-1)). The depth profile of the damage induced by the Al2+ ions was determined by Backscattering Spectrometry in channeling geometry (BS/C) with a 3.5 MeV He2+ beam. The BS/C spectra were evaluated using the RBX. code. The samples were subsequently annealed at 1100D C in N-2 for one hour, in order to analyze their structural recovery by BS/C and the amount of the remaining defects by means of Electron Paramagnetic Resonance (EPR). The results from the BS/C spectra corresponding to the as-implanted samples indicate that the damage depends strongly on the total fluence but also, although to a lesser extent, on the beam current density. The BS/C measurements reveal that all the samples, except the one implanted with the highest fluence, recover completely their original crystalline structure after the annealing. Furthermore the angular anisotropy of the EPR spectra indicates that the implanted region recovered a good crystallinity, although some residual defects were observed.

The defects at the 3C-SiC/SiO2 interface have been studied by X-band EPR spectroscopy in oxidized porous 3C-SiC. One interface defect is detected; its spin Hamiltonian parameters, spin S=1/2, C3V symmetry, g//=2.00238 and g⊥=2.00317, central hyperfine interaction (CHF) with one carbon atom and AB//[001]=48G and superhyperfine (SHF) interaction with three equivalent Si neighbour atoms and TB//[001]=12.4G, allow us to attribute the center to a sp3 coordinated carbon dangling bond center, PbC.

Knowledge about the creation and diffusion of intrinsic point defects is crucial for devising annealing strategies after irradiation steps as, e.g., implantation. Experimental information can be obtained by observing the appearance and/or disappearance of characteristic electrical, optical or magnetic spectra, however, these have to be first assigned to a given defect. In case of silicon carbide even this very first task has not been accomplished yet in case of the carbon vacancy, with which two different electron spin resonance (ESR) centers (anneling out at very different temperatures) have been identified. Ab initio all-electron supercell calculations have been carried out to determine the hyperfine constants of several defects in 4H-SiC in order to justify the models of the measured ESR signals. The quality of the results were tested on the well-documented case of interstitial hydrogen in silicon.

International audience; The effect of thermal treatments in nitric oxide (NO) on the paramagnetic defects at the 4H-SiC/SiO(2) interface are analyzed by EPR in oxidized porous samples. The results on ultrathin thermal oxides show that the NO treatment at 1000 degrees C is insufficient for an efficient reduction of the two dominant paramagnetic interface defects: P(bC) centers and carbon clusters. From the NRA and XPS analysis of bulk samples treated under the same conditions we attribute the weak effect to the low nitrogen concentration of only 1% at the interface.

Charge redistribution between two different photorefractive centers in CdTe:Ge allows for enhancement of two-beam coupling at 1.064 µm and 1.32 µm by the additional illumination of the sample. The gain factor reached at 1.064 µm without external electric field is only 20% smaller than the theoretical limit for CdTe. The "net" gain is obtained at 1.064 µm as well as at 1.32 µm and 1.3 times amplification of the signal wave is achieved at 1.06 µm with no applied field.